1. Field of the Invention
The present invention relates to an elongated structure for receiving light through an end or edge thereof, and radiating or diffusing the light out through a surface of the structure which is substantially perpendicular to the end or edge through which the light was introduced.
2. Description of the Related Art
A conventional optical fiber or light pipe is designed to transmit light introduced into one end thereof to an opposite end with minimum loss of light through the circumferential surface of the fiber. This is possible since light entering the end of the fiber at an angle less than the numerical aperture of the fiber is totally internally reflected throughout the length of the fiber to the other end.
However, it is desirable in many applications to adapt an optical fiber to radiate or diffuse light introduced into an end thereof from a point light source out through the circumferential or peripheral surface thereof, and thereby function as an elongated light source in combination with the point light source. This has been done in the past by grinding or otherwise roughening the circumferential surface of the optical fiber to reduce the reflectance thereof. Internally propagating light incident on the roughened surface of the fiber is not completely internally reflected, but is partially scattered out of the fiber through the surface. A partially transparent, or translucent optical fiber such as "opal" glass made of acrylic resin by the Asahi Chemical Co. of Japan also radiates light out the circumferential surface thereof.
Although useful in some limited applications, the expedients of making an optical fiber translucent, or roughening the circumferential surface of an optical fiber to produce light radiation out through the surface thereof, are generally unsatisfactory in that the intensity of the radiated light decreases exponentially from the input end along the longitudinal axis of the fiber.
U.S. Pat. No. 4,422,719, entitled "OPTICAL DISTRIBUTION SYSTEM INCLUDING LIGHT GUIDE", issued Dec. 27, 1983 to D. Orcutt, discloses a flexible light guide including a tube or sleeve of a smooth, flexible transparent or translucent plastic material which is shrink fitted onto a transparent, transmitting core. The external sleeve has an index of refraction N.sub.2 which is larger than the index of refraction N.sub.1 of the core, so that total internal reflection does not take place. The relationship between the indices of refraction of Orcutt's sleeve and core causes his light guide to radiate light out of its surface by refraction rather than by reflection.
The basic embodiment of Orcutt's light guide suffers from the disadvantage that the intensity of the radiated light decreases exponentially from the light source along the longitudinal axis of the fiber as described above. This also occurs in modified embodiments of Orcutt's light guide in which the surface of the sleeve is roughened, or granules are embedded in the sleeve.
Prior art optical fibers which do not produce a uniform distribution of radiated light along their lengths are not usable in practical applications such as back lights for displays, panel lights, and linear light radiators.
Orcutt further discloses an embodiment of his invention in which angular cuts are formed into the surface of the light guide sleeve. The cuts cause light propagating through the guide to be reflected outwardly through the sleeve. In order to provide more uniform brightness along the length of the guide, the cuts are made progressively larger as the distance along the axis from the light source increases, to reflect out a larger percentage of the remaining light traversing the core at each cut.
Although generally alleviating the problem of non-uniform light distribution, the cuts disclosed by Orcutt cause a substantial amount of light to be lost in the guide due to internal scattering. This precludes the use of such a light guide in applications in which a significant amount of loss is unacceptable.
A specific application in which highly uniform illumination is required is the newly emerging field of biosolar reactors such as described in an article entitled "Japanese Industry Going for `Green Technology`", published in Nature magazine, Vol. 350, 28 March 1991, pp. 266-267. Biosolar reactors include water-filled vessels into which genetically engineered marine bacterium or microorganisms are introduced. Sunlight is piped down into the vessels to enable the microorganisms to produce not only food products, but also hydrocarbon-based fuel products through photosynthesis.
A specialized application of a biosolar reactor, as described in the Nature article, is to mop up carbon dioxide emitted from power stations and industrial plants. Effluent air containing the carbon dioxide pollutant is bubbled through the reactor, and the carbon dioxide is assimilated by microorganisms known as synechococcus sp.
Early attempts to absorb significant amounts of carbon dioxide were confounded by the fact that the green-colored photosynthetic microorganisms readily attenuate incident light, and only the microorganisms within a few millimeters of the light source can grow efficiently. This problem was solved in accordance with the present invention by providing a closely packed stack of light-diffusing optical fibers embodying the invention which ensure uniform lighting and optimum growth of the bacterium throughout the vessel.